1. Technical Field
The invention relates to precision measurement instruments, and particularly to chromatic range sensors and similar optical distance determining devices, and their use.
2. Description of the Related Art
It is known to use chromatic confocal techniques in optical height or distance or range sensors. As described in U.S. Pat. No. 7,876,456 (the '456 patent), which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the surface height or distance relative to the focusing element determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. Upon reflection from the surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the aperture. All of the other wavelengths are poorly focused on the aperture, and so will not couple much power into the fiber. Therefore, for the light returned through the fiber, the signal level will be greatest for the wavelength corresponding to the surface height or distance to the surface. A spectrometer type detector measures the signal level for each wavelength, in order to determine the surface height.
Certain manufacturers refer to practical and compact chromatic range sensing (CRS) systems that operate as described above, and that are suitable for use in an industrial setting, as chromatic point sensors (CPS) or chromatic line sensors, or the like. A compact chromatically-dispersive optical assembly used with such systems is referred to as an “optical pen,” or a “pen.” The optical pen is connected through an optical fiber to an electronic portion of the chromatic point sensor which transmits light through the fiber to be output from the optical pen and which provides a spectrometer that detects and analyzes the returned light. The returned light forms a wavelength-dispersed intensity profile received by the spectrometer's detector array. Pixel data corresponding to the wavelength-dispersed intensity profile is analyzed to determine the “dominant wavelength peak pixel coordinate” as indicated by a peak or centroid of the intensity profile, and the dominant wavelength peak pixel coordinate is used with a lookup table to determine the distance to the surface. The dominant wavelength peak pixel coordinate may be determined with sub-pixel resolution, and may be referred to as the “distance-indicating pixel coordinate.”
Also known in the art is a CRS, which is a “line sensor” that uses a slit aperture and focuses light along a line rather than a point, providing the capability to measure a distance to a surface at a number of points along that line, as disclosed in U.S. Patent Application Publication No. 2010/0188742, which is hereby incorporated herein by reference in its entirety.